1. Field of the Invention
This invention relates to a thermosettable adhesive sealant or coating plastisol dispersion composition which, on heating at or above the fluxing temperature but below the flow temperature of the ABS, rapidly provides handling strength and which can be crosslinked to give a thermoset seal, bond or coating on further heating or irradiation.
The invention also relates to a process for forming a crosslinked bond, seal or coating.
2. Description of Prior Art
In general, a plastisol is composed of a high molecular weight polymer dispersed in a plasticizer which is a material incorporated in a plastic to increase its workability. Upon heating, the plastisol turns to a pregelled dispersion, to a gelled dispersion and then to a fused dispersion. The viscosity of a plastisol decreases with the increase of temperature at the beginning. At a certain temperature, suddenly, the viscosity increases sharply and the liquid dispersion turns to an opaque solid, a gelled dispersion. This temperature is called minimum fluxing temperature defined as the temperature at which a plastisol develops sufficient physical integrity to permit being lifted from the fusion plate. Upon further heating at a higher temperature, the plastisol turns to a clear plasticized plastic.
To prepare a plastisol, two basic ingredients, a high molecular weight polymer powder and a liquid plasticizer, are required to form a stable dispersion after blending. Physically, the plasticization process of a plastisol is the permeation of the plasticizer into the polymer particle to solvate the polymer molecules. The permeation rate (P) depends on the diffusion speed of the plasticizer (D) and the interaction between the plasticizer and the polymer (S): EQU P=D.times.S
Under the assumption that a polymer is compatible with a certain plasticizer, two important factors, the molecular weight (size) and the structure of polymer and plasticizer, should contribute to the stability of a plastisol which is determined by the diffusibility of the plasticizer upon aging. A stable dispersion should not allow the diffusion to occur at or below storage temperature. To prevent a plasticizer from diffusing, the size of the plasticizer molecules has to be larger than that of the polymer free volume. Upon heating, the free volume increases with temperature and allows the plasticizer molecule to diffuse into a polymer particle when the temperature is high enough.
Besides the kinetic process of plasticization, the capability of plasticization also depends on thermodynamic parameters. The plasticization should not occur when the free energy of mixing is greater than or equal to zero (.DELTA.G.sub.m .gtoreq.0), even if the size of the plasticizer is as ideal as described above.
Poly(vinyl chloride) and its copolymers, because of their low degradation temperature, eliminate hydrogen chloride and form a colored product below their melting temperature. Therefore, to use a melt process for these polymers without adding a plasticizer is difficult. The invention of plastisol technology has allowed these polymers to have excellent applicability and become the major polymers used in the plastisol industry. Unfortunately, the degradation of these polymers in service conditions is still an unacceptable problem in some applications due to hydrogen chloride elimination which promotes corrosion in metal and a reduction of polymer strength.
To stabilize PVC plastisols in service and to enhance their service life, a crosslinkable, secondary plasticizer has been incorporated with a primary plasticizer for plastisol preparation. The secondary crosslinkable plasticizer includes reactive vinyl compounds such as trimethylolpropane trimethacrylate and tetraethylene glycol dimethacrylate [Dainippon, JP80 52,335 (1980)]; G. F. Cowperthwalte, SPE Journal, 29, 56, 1973], unsaturated polyesters [Dainippon, JP80 21,474 (1980)], diallyl compounds [Shin-Nippon Rika, JP72 40,853 (1972)], and epoxy resins [Dunlop, JP81 100,841 (1981)].
To further improve the structural properties and eliminate the problem of hydrogen chloride release of the PVC plastisol, the plastisol technology was extended to acrylic polymers for the preparation of thermally fusible acrylic plastisols. See U.S. Pat. No. 4,125,700, which used esters as plasticizers and polyol acrylates as reactive diluents to prepare various reactive acrylic plastisols which formed a plasticized semi-interpenetrating network after crosslinking reactions.
U.S. Pat. No. 4,020,966 teaches a plastisol composition containing as a resin component a copolymer of a normal alpha-olefin and maleic anhydride in combination with a plasticizer and a reactive polyepoxide plasticizer.
The copolymer of acrylonitrile-butadiene-styrene (ABS) is an engineering plastic having extremely good impact resistance. The copolymer can be used to modify the impact resistance of another polymer such as polyvinyl chloride. See U.S. Pat. No. 3,969,469 and U.S. Pat. No. 4,259,460.
Using an organic solvent, methyl ethyl ketone, to dissolve an ABS and epoxy resin, an adhesive having improved chemical resistance, adhesion, flexibility and impact resistance was prepared as taught in U.S. Pat. No. 3,496,250. However, the use of a solvent generated problems and inconvenience. The dissolving of polymeric material is a time-consuming process, especially when the polymer has a high molecular weight. Furthermore, the use of a solvent leads to pollution and requires a long drying period. Also, the solvent-based adhesives containing high molecular weight polymer normally have a high viscosity which impedes processability unless the concentration is low. A blend containing ABS graft polymer and epoxy resin without solvent was also briefly described as being formed on a rubber mill which turns the composition into films. Unfortunately, the high shear mixer turns the mixture to a solid material which is not pumpable and can be applied as an adhesive or a coating only after remelt.
To prepare a pumpable thermosetting material, the instant invention discloses the dispersion of ABS powder in a reactive plasticizer such as liquid epoxy resin, liquid acrylic resin or the mixture of the two. This invention concerns the preparation of an ideal reactive plastisol which is a system containing a reactive or non-reactive polymer powder dispersed in a thermosetting plasticizer. The ideal reactive plastisol converts to a fused plastisol at minimum fluxing temperature, turns to a clear plasticized polymer at clear point and changes to a thermoset material after the crosslinking reaction. The characteristics of the reactive plasticizer thus include: wide compatibility with the polymer; low vapor pressure; high plasticization efficiency; excellent aging stability upon storage and crosslinkability upon curing.
This invention particularly concerns a class of reactive plastisols prepared from a copolymer of acrylonitrile-butadiene-styrene (ABS) and a reactive plasticizer or a combination of reactive plasticizers.
In reactive plastisol technology, which is a combination of plastisol and thermosetting technologies, the dispersion fuses into a plasticized solid at a temperature much lower than the melting point. Because the fluxing process is extremely quick, the thermosettable material provides a handling strength or B-stage strength in a few seconds. The final cure to a thermoset material can then be made to occur either by subsequent heating to the cure temperature or by irradiation, e.g., UV in the presence of a photoinitiator or by high energy ionizing radiation.